1. Field of the Invention
This invention relates to an electrode, a process for producing the same, and a battery using the same. More particularly, it relates to an elect-ode which increases its resistivity with increasing temperature, a process for producing the electrode, and a battery having the electrode.
2. Discussion of the Background
With the recent development of electronic equipment, batteries used therein as a power source have increasingly gained in capacity and output density. A lithium ion secondary battery is attracting attention as a battery fulfilling these requirements. A lithium ion secondary battery has an advantage of high energy density but requires sufficient safety measures because of the use of a non-aqueous electrolytic solution.
Conventionally proposed safety measures include a safety valve which relieves inner pressure increases and a PTC element which increases resistivity on heat generation due to an external short-circuit to shut off the electric current. For example, incorporation of a safety valve and a PTC element into the cap of a positive electrode of a cylindrical battery is known as disclosed in JP-A-4-328278. However, on the safety valve""s operation, moisture in the air enters the inside of the battery, which can induce an exothermic reaction in case lithium exists in the negative electrode.
On the other hand, a PTC element, which cuts off the circuit involving an external short, exerts no bad influence on operating. The PTC element can be designed to operate when the battery temperature rises to, for example, 90xc2x0 C. or higher due to an external short-circuit so as to be a safety element which operates first in case of abnormality.
Having the above-mentioned constitution, conventional lithium secondary batteries have the following problems.
When a short-circuit occurs in the inside of a conventional lithium secondary battery to raise temperature, the battery is incapable of suppressing an increase of the short-circuit current.
When a short-circuit occurs in the inside of a lithium secondary battery to raise temperature, a separator made of polyethylene or polypropylene interposed between a positive electrode and a negative electrode is expected to soften or melt to clog the pores of the separator, whereby the separator would exude the nonaqueous electrolytic solution contained therein or seal the nonaqueous electrolytic solution within itself to reduce its ion conductivity thereby to diminish the short-circuit current. However, the part of the separator distant from the heat generating part does not always melt. Besides, in case temperature rises, it is likely that the separator melts and flows to lose its function of electric insulation between positive and negative electrodes, which can lead to a short-circuit.
Further, the negative electrode of a lithium ion secondary battery is prepared by coating a substrate functioning as a current collector, such as copper foil, with a slurry comprising a negative electrode active material such as graphite, a binder such as polyvinylidene fluoride (PVDF), and a solvent, drying the coating layer to form a film. The positive electrode is similarly prepared as a film on a substrate functioning as a current collector, such as aluminum foil.
The positive electrode contains an active material, such as LiCoO2, a binder, and a conducting agent. The conducting agent is to enhance electron conductivity of the positive electrode in case where the active material has poor electron conductivity. The conducting agent includes carbon black (e.g., acetylene black) and graphite (e.g., KS-6).
When the temperature of such a battery increases over the temperature at which the separator melts and flows due to, e.g., an internal short-circuit, a large short-circuit current flows between the positive and negative electrodes at the part where the separator flows. It follows that the battery temperature further increases by heat generation, which results in a further increase of the short-circuit current.
The invention has been made in order to solve the above-described problem. An object of the invention is to provide an electrode which increases its resistivity with increasing temperature, a process for producing the electrode, and a battery having the electrode.
A first electrode according to the invention is an electrode comprising an active material and an electron conductive material that is in contact with the active material, characterized in that the electron conductive material contains a conductive filler and a resin and is constituted so as to increase its resistivity with increasing temperature. According to this invention, since the electron conductive material contains a conductive filler and a resin and is constituted so as to increase its resistivity with increasing temperature, the electrode is capable of suppressing an increase of electric current when temperature rises.
A second electrode according to the invention is characterized in that the resin of the electron conductive material has a melting point of 90 to 160xc2x0. According to this embodiment, since a resin having a melting point of 90 to 160xc2x0 C. is used in the electron conductive material, the electron conductive material increases its resistivity at around a predetermined temperature within a range of from 90 to 160xc2x0 C.
A third electrode according to the invention contains 0.5 to 15 parts by weight of the electron conductive material. With the electron conductive material content ranging from 0.5 to 15 parts by weight, the electrode has a reduced resistivity before the rate of change of electrode resistivity increases.
A fourth electrode according to the invention is characterized in that the proportion of the conductive filler in the electron conductive material is 40 to 70 parts by weight. The conductive filler content in the electron conductive material ranging from 40 to 70 parts by weight, the electrode shows a high rate of change in resistivity at around a prescribed temperature, and a battery having the electrode has an increased discharge capacity.
A fifth electrode according to the invention is characterized in that the electron conductive material has a particle size of 0.05 to 100 xcexcm. The particle size of the electron conductive material ranging from 0.05 to 100 xcexcm, the electrode has a reduced resistivity before the rate of change of electrode resistivity increases, and a battery having the electrode has an increased discharge capacity.
A sixth electrode according to the invention is characterized in that the conductive filler is a carbon material or a conductive non-oxide. Containing a carbon material or a conductive non-oxide as a conductive filler, the electrode has enhanced conductivity.
A seventh electrode according to the invention is characterized by containing a conducting agent which is to increase electron conductivity and hardly changes its resistivity with temperature. Since the electrode contains a conducting agent which is to increase electron conductivity and hardly changes its resistivity with increasing temperature, the resistivity of the electrode can be properly adjusted even in using an electron conductive material having low electron conductivity.
An eighth electrode according to the invention is characterized by containing at least two different kinds of electron conductive materials. Because at least two electron conductive materials different in kind are used, there is provided a high flexible electrode having a low resistivity at temperatures lower than a prescribed temperature. When the inner temperature of a battery having the electrode increases above a prescribed value, the electrode increases its resistivity to reduce the current flowing inside the battery, thereby improving the safety of the battery.
A ninth electrode according to the invention is characterized in that the electron conductive material contains at least two different kinds of conductive fillers. Because at least two conductive fillers different in kind are used, there is provided a high flexible electrode having a low resistivity at temperatures lower than a prescribed temperature. When the inner temperature of a battery having the electrode increases above a prescribed temperature, the electrode increases its resistivity to reduce the current flowing inside the battery, thereby improving the safety of the battery.
A tenth electrode according to the invention is characterized in that the electron conductive material contains at least two different kinds of resins. Because at least two resins of different kinds are used, there is provided an electrode having a low resistivity at temperatures lower than a prescribed temperature. When the inner temperature of a battery having the electrode increases above a prescribed temperature, the electrode increases its resistivity to reduce the current flowing inside the battery, thereby improving the safety of the battery.
An eleventh electrode according to the invention is characterized in that the active material is a cobalt-containing oxide. The active material being a cobalt-containing oxide, a battery having the electrode has a reduced current in case of a short-circuit.
A twelfth electrode according to the invention is characterized in that the active material is a manganese-containing oxide. The active material being a manganese-containing oxide, a battery having the electrode has a reduced current in case of a short-circuit.
A thirteenth electrode according to the invention is characterized in that the active material is an iron-containing oxide. The active material being an iron-containing oxide, a battery having the electrode has a reduced current in case of a short-circuit.
A fourteenth electrode according to the invention is characterized in that the resin is a crystalline resin. According to this embodiment, the rate of change of resistivity at around a predetermined temperature is increased by using a crystalline resin.
A first battery according to the invention comprises a positive electrode, a negative electrode, and an electrolytic solution provided between the positive and the negative electrodes, characterized in that the positive or negative electrode is any one of the above-described first to fourteenth electrodes. According to this structure, since the positive or negative electrode is any one of the above-described first to fourteenth electrodes, the electrode increases its resistivity when the temperature inside the battery increases above a predetermined temperature. As a result, the current flowing inside the battery is diminished, thereby bringing about improved safety.
A first process for producing an electrode according to the invention is characterized by comprising the steps of
(a) pulverizing an electron conductive material containing a conductive filler and a resin,
(b) dispersing the pulverized electron conductive material and an active material to prepare active material paste, and
(c) pressing the active material paste having being dried at a prescribed pressing temperature under a prescribed pressure.
Comprising the steps (a) to (c), the process secures good connections among the particles of the electron conductive material so that the resistivity of the electrode at temperatures lower than a prescribed temperature can be reduced.
A second process for producing an electrode according to the invention is the first process characterized in that the prescribed temperature is the melting point of the resin or thereabouts. By setting the prescribed temperature at or around the melting point of the resin, the connections among the electron conductive material particles are further improved so that the resistivity of the electrode at temperatures lower than the prescribed temperature can be reduced further.
A third process for producing an electrode according to the invention is the first process characterized in that the step of pulverizing an electron conductive material containing a conductive filler and a resin is carried out by making the electron conductive material to collide with a wall or with each other in an ultrasonic stream. The electron conductive material can be pulverized into small particles by collision with a wall or with each other in an ultrasonic stream. An electrode produced by using the thus pulverized electron conductive material has further reduced resistivity at temperatures lower than a prescribed temperature.
A fourth process for producing an electrode according to the invention is the first process characterized in that the step of pulverizing an electron conductive material containing a conductive filler and a resin is carried out by applying a combination of shear force, frictional force, and impact force to the electron conductive material. The electron conductive material can be pulverized into particles with reduced size variation by applying a combination of shear force, frictional force, and impact force to the electron conductive material. An electrode produced by using the thus pulverized electron conductive material has high flexibility and is therefore easy to fabricate.
A fifth process for producing an electrode according to the invention is the fourth process characterized in that the electron conductive material is pulverized under cooling. In this embodiment, an electron conductive material with further reduced size variation can be obtained by carrying out the pulverization while cooling. An electrode produced by using the thus pulverized electron conductive material has still higher flexibility and is therefore still easier to fabricate.